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Viruses
Are Viruses Living Things?Characteristics of Living things are…• Made of cells• Can reproduce• Based on a universal genetic code• Grow and develop• Obtain and use materials and energy• Respond to the environment• Maintain homeostasis• As a group, change over time
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Viral Characteristics
•NON-LIVING
•REPRODUCE ONLY WITHIN A HOST CELL by hijacking the cells’ machinery
•NAMED FOR DISEASE THEY CAUSE OR PLACE THEY INFECT
Which type of cell will be infected by the virus shown below?
Name tells you this virus infects bacteria
Name tells you this virus causes influenza
Viral SizeViral Size
• Viruses are very small. If an average virus were the size of an average person, a bacterial cell would be the size of a dinosaur over ten stories tall.
Click on the picture for virus size activity
Whiteboard Activity
Cell, Virus or Both
Activity: Cell, Virus, or Both?
Contains nucleic acid as genetic material
Both
Activity: Cell, Virus, or Both?
Can grow and develop
Cell
Activity: Cell, Virus, or Both?
Can only replicate in a host
Virus
Activity: Cell,Virus, or Both?
Can reproduce on its own
Cell
Activity: Cell, Virus, or Both?
Obtain and use materials and energy
Cell
Activity: Cell, Virus, or Both?
Respond to the environment
Cell
Activity: Cell,Virus, or Both?
Maintain homeostasis
Cell
Activity: Cell, Virus, or Both?
Can contain DNA
Both
Activity: Cell, Virus?
Which is smallest?
Virus
Activity: Cell, Virus, or Both?
Cell
Activity: Cell, Virus, or Both?
Virus
Viral Structure
• All viruses have:– genetic material (DNA or RNA)– Capsid: protein coat that enables a virus to enter a
living cell
How do viruses infect cells?
• Once a virus enters a host cell, two different processes can occur:
– Lysogenic cycle
– Lytic cycle
Lytic Cycle
1. viral DNA is injected into the host cell2. host cell begins to make mRNA from
the viral DNA3. viral proteins shut down the host cell’s
regular functions4. cell is used to make thousands of
viruses (DNA and protein coats)5. host cell bursts (lyses) and releases
viruses to infect other cells
Lysogenic Cycle
Lysogenic Cycle
1. viral DNA is injected into the host cell
2. DNA is integrated into the host DNA
3. host cell divides with the viral DNA as a part of it
4. eventually the viral DNA can be triggered to separate from the host cell DNA and pick up with the lytic cycle at step 2.
• Turn to the person next to you and discuss the difference between the lytic and lysogenic cycles.
How do RNA viruses work?
• RNA viruses do not have built in proofreading, so they mutate and evolve quickly (like the flu).
• A special kind of RNA virus, called a retrovirus, can cause DNA to be made from their RNA. – Certain cancers, and AIDS are caused by
retroviruses.
